Hepatitis C virus is the most common cause of chronic viral hepatitis within the United States, infecting approximately 4 million Americans and responsible for the deaths of 8,000-10,000 persons annually due to progressive hepatic fibrosis leading to cirrhosis and/or the development of hepatocellular carcinoma. Hepatitis C virus is a single stranded, positive-sense RNA virus with a genome length of approximately 9.6 kb. It is currently classified within a separate genus of the flavivirus family, the genus Hepacivirus. The hepatitis C virus genome contains a single large open reading frame (ORF) that follows a 5′ non-translated RNA of approximately 342 bases containing an internal ribosome entry segment (IRES) directing cap-independent initiation of viral translation. The large ORF encodes a polyprotein which undergoes post-translational cleavage, under control of cellular and viral proteinases. This yields a series of structural proteins which include a core or nucleocapsid protein, two envelope glycoproteins, E1 and E2, and at least six nonstructural replicative proteins. These include NS2 (which with the adjacent NS3 sequence demonstrates cis-active metalloproteinase activity at the NS2/NS3 cleavage site), NS3 (a serine proteinase/NTPase/RNA helicase), NS4A (serine proteinase accessory factor), NS4B, NS5A, and NS5B (RNA-dependent RNA polymerase).
With the exception of the 5′ non-translated RNA, there is substantial genetic heterogeneity among different stains of hepatitis C virus. Phylogenetic analyses have led to the classification of epatitis C virus strains into a series of genetically distinct “genotypes,” each of which contains a group of genetically related viruses. The genetic distance between some of these genotypes is large enough to suggest that there may be biologically significant serotypic differences as well. There is little understanding of the extent to which infection with a virus of any one genotype might confer protection against viruses of a different genotype.
Several types of human interferon have proven effective in the treatment of infection by hepatitis C virus, either alone as monotherapy, or in combination with ribavirin. However, treatment with interferon-ribavirin carries a high risk of treatment failure, either primary failure of virus elimination, or relapse of the infection upon cessation of therapy. Moreover, these therapeutic agents are relatively toxic and are associated with a high frequency of adverse reactions. The development of better (more effective and safer) antiviral agents capable of suppressing or eliminating hepatitis C virus infection has been hindered by the fact that this virus replicates with very low efficiency, or not at all, in cultured cells. The absence of a highly permissive cell culture system that is capable of supporting robust replication of the virus has prevented the development of high throughput antiviral screens for use in the development of inhibitors of viral replication, and has delayed the investigation of the virus and relevant aspects of its molecular and cellular biology. It has also stymied efforts at vaccine development and the immunologic characterization of the virus, the human response to hepatitis C virus, and the diseases associated with infection. The development of infectious molecular cDNA clones of the viral genome has done little to solve this problem, since virus can be rescued from the RNA transcribed from such clones only by its injection into the liver of a living chimpanzee or other susceptible primate.